(1) Field of the Invention
This invention relates to an X-ray detector for measuring X rays in the medical, industrial, nuclear and other fields.
(2) Description of the Related Art
In an X-ray detector having electrodes formed at opposite sides of a semiconductor, a predetermined bias voltage is applied between the electrodes, and electric charges generated in the semiconductor by incident X rays are detected as electric signals. For such an X-ray detector, various semiconductor materials are selectively used according to purpose. These semiconductor materials are manufactured in various ways. Generally, for an X-ray detector required to have energy resolution, a high-purity single crystal semiconductor such as silicon (Si) tends to be used.
An X-ray detector using amorphous selenium (a-Se) in particular can easily realize a high resistance thick film sized 1,000 cm2 or larger by using a film coating technique such as vacuum deposition method. This X-ray detector is ideal for use in a field requiring a large area for X-ray measurement.
However, an amorphous selenium (a-Se) film formed by such a method includes many structural defects. Generally, therefore, an appropriate quantity of impurity is added (i.e. doped) in order to improve performance.
The conventional detector constructed as described above has the following drawback.
Unlike a single crystal semiconductor, the conventional detector has many potential structural defects. These defects trap charge transfer media (carriers) of electrons and holes generated in the semiconductor layer by X-ray incidence. The trapped carriers cannot be picked up as electric signals. This causes a phenomenon of deterioration in the sensitivity of the X-ray detector.
This phenomenon will particularly be described hereinafter with reference to FIGS. 2A and 2B. FIGS. 2A and 2B are explanatory views of an internal structure of the X-ray detector. The structural defects in amorphous selenium (a-Se), as shown in FIG. 2B, include recombination centers D0 and ionized recombination centers D+ (electron trapping centers) and D− (hole trapping centers) present in a fixed ratio. A density of D+ and D− at this time determines an initial value of sensitivity of the X-ray detector. This state is expressed by the following formula:2D0→D++D−
When X rays impinge in this state to generate charge transfer media (carriers) of electrons (e−) or holes (h+) in amorphous selenium (a-Se), these media are first trapped by the recombination centers D0 to change into D− and D+, respectively. In this way, the density of D+ and D− increases to deteriorate sensitivity. This relationship is expressed by the following two formulas:D0+e−→D−D0+h+→D+